High and low pressure turbine engine components like vanes, stators, and rotor blades are made of nickel based superalloys. Typically, these components are protected from the high temperature environment by a thermal barrier coating (TBC). However, the coating can be damaged due to oxidation, corrosion, and/or erosion during service, requiring scheduled repairs or being scrapped if material loss has thinned down the wall of the structure below allowable limits.
Traditional repair methods entail removing the existing coatings and apply new coatings to the engine components. The repair process generally causes material loss of the base metal. As the wall thickness approach allowable limit as a result of repair, the engine parts can no longer be reused. Therefore, dimensional restoration in engine repair service can lead to economic gain and reduce the amount of scrap parts that still have substantial remaining material value.
One of the current practices of engine repair is to deposit nickel (Ni) onto the damaged parts followed by a high temperature diffusion process to convert the nickel deposit to a desired alloy composition. While diffusion of chromium (Cr) into the Ni deposit layer can enhance the high temperature oxidation resistance of the repaired part, the diffusion process can gradually consume the chromium (Cr) and other minor compositions from the parent parts, i.e., vanes.
Since the major composition of the vanes is Ni and Cr, plating a Ni—Cr alloy to satisfy the composition requirement can greatly retard or even reverse the depletion of the Cr from the parent parts. Thus, Ni—Cr deposit is attractive to enable engine dimensional restoration.
Electrodeposition is a non-light-of-sight coating application technique suitable for the parts with complex geometry, such as engine vanes and airfoils. Electrodeposition of Ni—Cr alloy in traditional plating chemistry has not been successful in forming a deposit thick enough for the structural repair (>125 μm) with dense structure. The challenge is suspected to be related to the inability to deposit thick Cr deposits greater than 10 μm from conventional aqueous trivalent chromium plating baths.
Although thick hard chromium has been produced in hexavalent chromium solution, i.e. chromic acid, the hard chromium deposit has cracks and hexavalent chromium is highly carcinogenic. Therefore, it is desirable to develop plating chemistry using only trivalent chromium as the Cr source to produce Ni—Cr alloys for the engine dimensional restoration applications.